This invention relates to a method for manufacture of large tanks according to the "from the top and down" principle, whereby the roof of the tank is manufactured first at ground level, and successive generally cylindrical plates are attached one below the other as the completed part of the tank is raised in stages, the tank thereby growing from below.
At present there are in principle two usable methods for rational manufacture of large tanks on site. According to one of these methods, called "from the bottom and up", the bottom plate is laid out and welded, the lowermost plate sweep (i.e., generally cylindrical plate section) of the tank is mounted to the bottom plate and welding is carried out in the vertical seams and to the bottom plate. The next plate sweep is placed on the one beneath it and welded to this, whereupon the vertical seams are welded. The supporting structure of the roof consists as a rule of a center ring and radial, somewhat curved steel girders which are suspended on the tank wall and in the center ring. The center ring is placed at the right height on a temporary scaffolding and the steel girders are placed in the right position and welded to the center ring and to the wall. When this task has been completed the tank structure is self-supporting and the temporary scaffolding for the center ring can be dismantled, whereupon the roof covering of plate is mounted and welded.
According to the other prior art method, called "from the top and down", the bottom plate is laid out and welded. The plate sweep lying uppermost in the finishing tank is mounted and the vertical seams are welded. The roof of the tank is manufactured in principle in the same manner as in the previously described method but largely at ground level and in some cases small openings must be left in the roof for the hoisting equipment used to raise same. The hoisting devices are then mounted and connected via lugs to the topmost plate sweep, whereupon the finished tank section is lifted by the hoisting devices disposed around the plate sweep a height corresponding to the height of the next plate sweep. The next topmost plate sweep in the finished tank is then mounted below the raised completed section and vertically and horizontally welded to the lowermost plate sweep of the raised completed section. The lifting devices are then connected to the plate sweep which has just been assembled and the above procedure is repeated until the tank has attained its full height. The bottommost plate sweep is then welded to the bottom plate.
In welding work according to the first of the prior methods mentioned above, a working platform has to be installed on a working level with each plate sweep, which is a time-consuming and risky operation. When the other prior method is used, the welding work is done at ground level and usually without a working platform. Automatic welding machines have been used for some 20 years in connection with the first above-mentioned prior method and in this context use is made of a welding machine which is hung up on wheels on the upper edge of the topmost plate sweep, the wheels being driven synchronously with the desired welding speed. For the vertical seams a welding machine is used which is either suspended on the topmost plate sweep or mounted on a vertical pillar which is successively lengthened as the tank-building work proceeds. Important in all welding work is that the automatic welding machines have to be guided or steered exactly parallel to the welding seam and driven synchronously with the welding speed. In tank building according to the prior "from the top and down" method, two methods of automatically welding horizontal seams have been tested and pillar welding machines are relatively often used for the vertical seams. According to one of these prior horizontal welding methods the welding machine was suspended by wires which were attached to a trolley which ran on a track on the inside of the tank roof. As the tank was successively raised in stages the wires had to be lengthened and consequently difficulties were experienced in getting the trolley and the welding machine to run synchronously relative to each other. An attempt was then made to solve this problem by replacing the wires by a pipe scaffolding but the same problem arose as soon as the distance between the welding machine and the suspension track became large. For this reason this prior method did not attain any great sucess.
According to the second prior horizontal welding method the welding machine was hung up in a scaffolding with a drive device running on a track which was mounted on the bottom plate of the track and with idler wheels against the tank wall. Since tank bottoms are often not flat and to some extent may also be dished or wave-shaped and since the tank wall is seldom perfectly circular, difficulties were experienced in keeping the electrode of the welding machine in the correct position relative to the seam.
From the above it follows that there is currently no practical method by which to automatically weld the seams in the tank walls when the tank is built by the "from the top and down" method and a primary objective of the present invention is to provide a suitable method for this.
The hoisting devices used when building tanks from the top and down usually comprise hydraulic lifters which cooperate with a hoisting pillar so that the lifters pull or press an arm which is obliquely directed upwards against the tank wall. The connection between the smooth tank wall and the tops of the inclined arms is achieved in that lugs are welded to the tank wall, the tops of the inclined arms working against the lower part of the said lugs. These lugs are welded to the plate sweep at which lifting is to occur and exactly equidistant from the lower edge of the plate sweep.